This invention relates to preorientation of the magnetic particles of and premagnetization of magnetizable media, particularly wide magnetic tapes.
As used herein the term premagnetization refers to the state of a magnetizable medium being initially magnetized substantially to saturation in a selected direction prior to the medium being employed for recording of information. As such, premagnetization preferably would be effected prior to each instance of information being recorded thereon. The term information as used herein constitutes latent magnetic images generated in the magnetizable layer of the magnetic medium, which images are, for example, composed of numerous minutely sized magnetic zones called pixels, arranged to form say characters. Each pixel constitutes for instance a 4-mil by 4-mil zone wherein the initial direction of magnetization, i.e., the premagnetization condition, has been reversed by an external magnetic field. The direction of magnetization preferred and specifically contemplated herein is the so-called transverse direction, i.e., the direction perpendicular to the intended direction of movement of the medium past a recording head (or perpendicular to the longitudinal axis or side edge in the case of a magnetic tape) but in the plane of the medium. Thus the present invention relates in particular to the repetitive creation, in magnetic media, especially in wide magnetic tapes, of the state of substantially saturated transverse premagnetization.
As used herein the term preorientation refers to the act of physically effecting the substantially unidirectional oriention or alignment of the magnetic particles or crystals of the magnetizable medium while the magnetizable layer is in the slurry state during the formation of the medium. Most of the commercially desirable materials utilized in forming the magnetic layer, such as chromium dioxide (CrO.sub.2), are comprised of elongated magnetizable particles or crystals. Generally these particles exhibit their best operative magnetic characteristics (e.g., magnetic remanence and "squareness") if they are magnetized by an external field having the direction of its magnetization parallel to the long axis of the magnetizable particles or crystals. Thus, the present invention relates in particular to the preorientation of the magnetizable particles of magnetic media, especially wide magnetic tapes, in the contemplated operative transverse direction. For a more complete understanding of the above-discussed aspects, the reader is referred to one such premagnetization operation using a preoriented magnetic tape as described in U.S. patent application Ser. No. 268,595, filed May 29, 1981.
Generally, for relatively narrow magnetic media (e.g., magnetic tapes around three inches or less in width) the magnetic structures presently employed are sufficiently small, compact and/or light weight to be well adapted for most applications of premagnetizing. For relatively wide magnetic media (e.g., tapes with width&gt;three inches), however, substantially larger permanent magnets are needed to span the greater width, which are impractical for gap dimensions in the range of six inches and larger. Although these large permanent magnets generally operate satisfactorily in substantially uniformly premagnetizing such wide tapes, they are undesirably bulky and heavy and relatively very expensive. As such they are commercially unattractive.
Generally an effective geometry for a magnetizing structure in terms of maximizing field strength is a u-shaped or preferably a c-shaped structure. Moreover, a pair of such structures placed in opposition of one another with the magnetizable medium arranged to be movable therebetween within the pole gaps is a most efficient way of providing a uniform magnetization of the medium, with virtual elimination of so-called vertical components.
For longitudinal premagnetization, an u-shaped structure with elongated pole faces may be readily utilized, inasmuch as the structure is simply arranged transverse to the direction of the medium's movement. As such, the structure is merely made as long as needed to span the entire (or desired) width of the medium. With such a structure orientation, longitudinal premagnetization can be readily effected with a small-gap magnetizing structure, and as such the structure can be of relatively low weight and inexpensive.
The situation becomes quite different, however, for transverse premagnetization, in that such a structure would have to be arranged longitudinally, i.e., arranged lengthwise in the intended direction of tape movement to achieve the desired transverse direction of magnetization. For a narrow tape, a small-gap structure may still be effectively utilized, since the gap should be large enough to accept the entire width (or desired portion of the width) of the narrow tape (e.g., one-half inch) to uniformly premagnetize the tape by itself. On the other hand, utilization of such a small-gap structure to provide transverse magnetization would enable one to magnetize only a small portion of the total width of a wide medium. Moreover, nothing is gained by the elongated pole faces of the above-mentioned structure with such an orientation.
In the case of wider tapes, (e.g., in excess of three inches), it thus follows that for single-structure uniform premagnetization, the gap of such structure must remain as great as or greater than the width of the medium (or the desired portion thereof). The result is that it becomes commercially impractical to provide single-structure arrangements to premagnetize wider mediums transversely, since such structures would necessarily be quite costly, bulky and heavy. This may be particularly appreciated in connection with such applications as office printers. It will be appreciated too, that a single-structure arrangement is particularly disadvantageous for applications employing endless "web" mediums of say eleven inches or more.
The above considerations are compounded when it is desired to have a pair of such structures arranged to oppose one another on either side of the medium for eliminating vertical magnetic components. That this approach is commercially prohibitive can be readily gathered from the considerations that the cost of such an arrangement would be in the hundreds of dollars, its size would be entirely impractical, and its weight would be excessive, (most probably in the range of several hundreds of pounds).
What is needed and would be useful, therefore, is an apparatus which would be capable of uniformly premagnetizing relatively wide magnetic tapes and yet would eliminate the above-mentioned undesirable features, and such is a primary objective of this invention.
With regard to preorientation of magnetic tapes, particularly wide tapes, and especially so-called high coercivity media, present needs are served with relatively very large, heavy and costly, specially designed magnet arrangements. This is so essentially because relatively very large magnetic fields are generally needed to preorient such materials with any appreciable commercial rate of throughput. More specifically, the magnitude of the external magnetic field needed to preorient the magnetic particles is a function of the magnetic material of the medium itself (i.e., the coercivity level, the concentration of magnetic pigment, etc.), the viscosity of such material in the slurry state during preorientation, and the rate of throughput. The faster such rate is, for example, the higher the field that is needed to substantially uniformly align the magnetic crystals. On the other hand, the more viscous the material the longer it takes for the magnetic particles to align, and thusly the higher the influencing magnetic field which is needed to increase the overall rate at which alignment is achieved. Therefore, in order to have the viscosity at manageable levels in a commercial process, the aligning magnetic arrangement must be placed as close to the slurry deposition apparatus as practicably possible. However, because of the relatively great size of such magnets, one only position the magnet just so close to the deposition apparatus. To compensate for this the slurry mixture would need to be provided with a greater amount of solvent or a slower evaporating solvent to avoid the magnetic pigmented slurry from becoming too viscous before fully reacting to and leaving the magnetic preorienting field and station, which compensating factor tends to slow the formation process down appreciably.
What is needed and would be useful, therefore, is a magnetic field-producing apparatus providing a sufficient external magnetic field to substantially uniformly preorient the magnetic particles of the slurry that comprises the magnetizable layer of a magnetic medium, which apparatus is relatively small in size and lightweight and, in terms of its external magnetic field, thus may effectively be positioned appreciably closer to the deposition apparatus, thereby tending to relieve the concern regarding solvent/viscosity/time criticality, and such is another principal objective of this invention.